1. Field of the Invention
The present invention relates generally to ball valves, and more particularly, but not by way of limitation, to an improved bearing retainer and method of assembly for a trunnion ball valve.
2. Brief Description of Related Art
In the typical construction of a trunnion mounted ball valve, the ball is machined to provide “trunnions” that are mounted in bearings. The bearing-trunnion combination is intended to support the ball in a stationary position relative to the flow path, but allow rotation of the ball between an open position and a closed position. The ball engages a pair of seats to form a seal around the ball. The valve is sealed as a result of the upstream valve seat moving against the ball in response to the line pressure. This is in contrast to a floating ball valve where the ball moves along the flow path and seals against the downstream valve seat as a result of the pressure applied to the ball.
A variety of designs for trunnion type valves exist. Notable design differences include the manner in which an operating stem, used for rotating the ball, is assembled and retained. The stem can be inserted internally from within the body cavity. This is known as a “blow-out-proof” design because the stem cannot be removed without disassembling the valve. Another option is to assemble the stem externally and retain it with bolted glands or pins. Another notable difference is that the trunnions can be assembled internally or externally. When assembled externally they are commonly retained with bolted glands. Alternatively, the trunnions can be machined directly on the ball. The ball-trunnion combination can then be inserted internally along with a bearing retainer.
In designing a trunnion valve, the overall length is determined by industry accepted standards. Therefore, the body section of the valve is the only area that is subject to original design. To produce a valve which can compete economically in the market requires a design that minimizes the overall body size and total weight of materials used to produce the valve body.
The different types of valve construction noted above are the result of trading one feature to gain another with the usual result of a less than optimal design. For example, the “blow-out-proof” stem design is generally considered preferable because it cannot be accidentally removed under pressure. Its simplicity also makes it less expensive to produce when compared with all the extra pieces needed to retain an externally mounted stem. Unfortunately, the “blow-proof” stem is not often used because it requires the overall diameter of the valve body to be increased to allow enough room for the ball to be assembled with the stem protruding into the bore. The result of an increase in total weight negates any design savings because the body material is more expensive than the cost of the eliminated parts. The choice of trunnion design also has important tradeoffs. The external pins, bearings, and means of retention are expensive to produce but require a much smaller overall valve body diameter. Internal bearing retainers are simpler and less expensive to produce but generally require a considerable gap between the ball and the body. As a result, a much larger overall body diameter is needed.
A bearing retainer is used to retain a bearing through which the trunnion extends. The bearing retainer is also known as a “trunnion support” or a “trunnion block.” Some designs have modified the internal bearing retainer in an effort to minimize the necessary gap. For example, the bearing retainer can be provided with longitudinal edges that are shaped to better conform to the contour of the ball cavity of the body section. This reduces the gap considerably if an externally loaded stem is used. With an internally loaded stem design, the upper bearing retainer may be slotted on one side to permit the bearing retainer to be moved past the lower end of the stem during assembly of the ball and bearing retainer. However, a slotted bearing retainer reduces the bearing support area of the bearing retainer along the direction of the fluid flow. In turn, a larger bearing retainer and thus a larger valve body is often required to provide sufficient bearing support, thereby obviating the purpose of the slotted bearing retainer.
To this end, a need exists for an improved bearing retainer and method of assembly which permits a particularly small gap between the body and the ball while maintaining internal trunnions in a “blow-out-proof” stem and without sacrificing bearing load strength. It is to such an improved bearing retainer and method that the present invention is directed.